1. Field of the Invention
The present invention relates to an engine ignition control apparatus and a method for controlling ignition of cylinders of a 4-cycle engine. More particularly, the present invention relates to an ignition control apparatus having fewer ignition coils than a number of cylinders of the engine, for controlling ignition of the cylinders during engine operation.
2. Description of the Background Art
In a four cylinder 4-cycle engine, for example, a first cylinder and a fourth cylinder are mechanically operated at a same phase (e.g., a first phase) and at timings opposite to each other with respect to strokes. In this type of engine, when the first cylinder is at a compression top dead center position, the fourth cylinder is at an exhaust top dead center (TDC) position. In the same manner, the second cylinder and the third cylinder are also operated in coordinated fashion at another phase (e.g., a second phase) and at timings opposite to each other.
An example of a known technique which ignites four cylinders of the 4-cycle engine using two ignition coils, i.e., using fewer ignition coils than number of cylinders, is disclosed in the Japanese patent publication 3314390.
According to the Japanese patent publication 3314390, the first system consisting of the first and fourth cylinders, and the second system consisting of the second and third cylinders respectively share one ignition coil in common. The ignition spark is generated every time one cylinder (of one of the systems) arrives at the TDC ignition timing position for each of the systems. Here, although the other cylinder is also ignited, the other cylinder is at (or may be at) an exhaust top dead center thereof. Hence, when the cylinder is at an exhaust top dead center, such ignition is treated as a “waste” fire.
In order to ignite all four cylinders of the engine efficiently, it is desirable to set the ignition timing for each cylinder. However, in an ignition method which shares a single ignition coil in common for each of the first and second systems as disclosed in the Japanese patent 3314390 publication, two cylinders which belong to the same system (i.e., the first system or the second system) are controlled based on the same ignition map.
Here, a lead angle limit of ignition timing differs for each of the cylinders, and it is necessary to avoid setting of the ignition timing which exceeds the lead angle limit. Accordingly, it is necessary to set the ignition timing for each of the first and the second systems in accordance with a cylinder having the lowest lead angle limit for the system. Hence, it is not possible to sufficiently increase a lead angle of the cylinder which has a higher lead angle limit.
Further, in order to overcome the above-mentioned technical drawback, the preparation of an ignition map for each of the cylinders (e.g., first through fourth ignition maps corresponding to respective one of the first through fourth cylinders) is considered. However, in such a case in which the ignition timing is prepared for each of the cylinders, it is impossible to discriminate whether the top dead center is a compression top dead center or an exhaust top dead center until a stroke determination is completed, even when each cylinder arrives at a top dead center thereof. Accordingly, cylinders which belong to the same system are ignited at the same ignition timing which conforms to the cylinder having the low lead angle limit, and the respective cylinders are ignited at independent ignition timings after the stroke determination.
For example, when the lead angle limit of the fourth ignition map is higher than the lead angle limit of the first ignition map, until the stroke determination is completed, both of the first and fourth cylinders are ignited at the ignition timing of the first ignition map, and after the stroke determination, the first cylinder is succeedingly ignited at the ignition timing of the first ignition map and the ignition timing of the fourth cylinder is changed over to the ignition timing of the fourth ignition map. However, this control method which changes over the ignition map before and after the stroke determination has following technical drawbacks.
In other words, to explain the technical drawbacks by focusing on the second and third cylinders which belong to the second system, between a point of time that the timing of system ignition is calculated at a predetermined calculation stage and a point of time that the ignition is actually performed at the ignition timing, it is necessary to ensure a time and the like in which electricity is supplied to the ignition coil and the ignition energy is stored. Accordingly, there may be a situation in that the stroke determination is completed after calculating the ignition timing of the system ignition and before ignition at the ignition timing.
On the other hand, a timer for measuring the ignition timing is provided on a one-timer-for-one-ignition-coil basis. Hence, when an independent ignition control is started immediately after the completion of the stroke determination, for example, the third cylinder arrives at the calculation stage immediately after the stroke determination, and the ignition timing is calculated based on the third ignition map, there may be a situation in which a predetermined value of the ignition timer is rewritten to the ignition timing of the third cylinder in independent ignition from the ignition timing of the second system ignition.
Accordingly, there has been a technical drawback that when the ignition timing of the second cylinder comes earlier than the ignition timing of the third cylinder, the second cylinder is ignited at the ignition timing calculated based on the third ignition map.
The present invention has been made to overcome such drawbacks of the existing ignition control apparatus. Accordingly, it is a first object of the present invention to provide an engine ignition control apparatus which can independently control ignition timings of different cylinders, by providing ignition maps dedicated to respective cylinders using only two ignition coils in a four cylinder 4-cycle engine.
Also, it is a second object of the present invention to provide an engine ignition control apparatus which can prevent an erroneous ignition of a cylinder based on an ignition map of another cylinder, even when the ignition is changed over from the system ignition based on two ignition maps to the independent ignition based on four ignition maps before and after the stroke determination using only two ignition coils in a four cylinder 4-cycle engine.